Electronic displays can use transmissive or emissive materials to generate pictures or light. Emissive materials are usually phosphorescent or electroluminescent materials. Examples are inorganic electroluminescent materials such as applied in thin film and thick film electroluminescent displays (EL-displays, for example thin film TFEL displays as manufactured by Sharp, Planar, LiteArray or iFire/Westaim) or light emitting diodes (LEDs). Another group is organic electroluminescent materials (such as Organic Light Emitting Diode or OLED materials) deposited in layers comprising small molecule or polymer technology or phosphorescent OLED, where the electroluminescent materials are doped with a phosphorescent material. Yet another group of materials are phosphors, commonly used in the well-established cathode ray tubes (CRT) or plasma displays (PDP) and even in emerging technologies like laser diode projection displays where the laser beam is used to excite a phosphor imbedded in a projection screen.
Two basic types of displays exist: fixed format displays which comprise a matrix or array of “cells” or “pixels” that are individually addressable, each producing or controlling light over a small area, and displays without such a fixed format, such as scanning electron beam displays, e.g. a CRT display. Fixed format relates to pixelation of the display as well as to the fact that individual parts of the image signal are assigned to specific pixels in the display.
Tiled displays may comprise modules made up of tiled arrays which are themselves tiled into supermodules. Modular or tiled emissive displays, such as e.g. tiled LED or OLED displays, are made from smaller modules or display boards that are then combined into larger tiles. These tiled emissive displays or display tiles are manufactured as a complete unit that can be further combined with other display tiles to create displays of any size and shape.
All light emitting elements on display boards and display tiles can be formed from different batches, can have different production dates, different run times, etc, i.e. they can have different properties. In the factory, i.e. before real use, all light emitting element products are calibrated under controlled circumstances. However, there is one parameter which can only be compensated based on statistical data and not on actual data, and that is the aging or degradation of the light emitting elements when they are being used. Age differences occur, for example, due to the varying ON times of the individual light emitting elements (i.e., the amount of time that the light emitting elements have been active) and due to temperature variations within a given display area.
For large-screen applications, where the display may consist of a set of tiled display boards, there is the possibility that one display will age at a faster rate than another, because of varying ON times of its light emitting elements and/or because of temperature differences. Typically, when a tiled display is manufactured, it is calibrated for a uniform image. The challenge in a display comprising light emitting elements is to make its light output uniform, i.e. to make all light emitting elements on the display board to have the same brightness, even after having been used.
In EP 1 158 483 a system 10 is described which corrects for the aging of the pixels in a display. The system 10 comprises a solid-state display device 12. The system 10 uses reference pixels 14 to enable the measurement of pixel performance and a feedback mechanism responsive to the measured pixel performance to modify the operating characteristics of the display device 10 (see FIG. 1). The characteristics of the reference pixels 14 are measured by a measurement circuit 18 and the information gathered thereby is connected to an analysis circuit 20. The analysis circuit 20 produces a feedback signal that is supplied to a control circuit 22. The control circuit 22 modifies the operating characteristics of the image display 10 through control lines 24.
According to EP 1 158 483, the measurement circuit 18 monitors the performance of the reference pixel 14. The measured performance values are compared to the expected or desired performance by the analysis circuit 20. These comparisons can be based on a priori knowledge of the characteristics of the device 12 or simply compared to some arbitrary value empirically shown to give good performance. In either case, once a determination is made that the performance of the device 12 needs to be modified, the analysis circuitry 20 signals the feedback and control mechanism which then initiates the change.
In the system 10 according to EP 1 158 483, however, errors in the measurement circuit 18 can lead to errors in the correction or change. Furthermore, the value the measured performance values are compared to is not exactly measured under the same circumstances as the measured performance values and thus can include small deviations from a reference value which would be measured under the same circumstances as the performance value. This could lead to errors in the correction or change.